1. Field of the Invention
This invention relates to wire and cable. In one aspect the invention relates to power cables that comprise a core element surrounded by a semiconductive shield which in turn is surrounded by an insulation layer which in turn is surrounded by a semiconductive insulation shield layer while in another aspect, the invention relates to power cables in which at least one of the semiconductive shields is strippable from the insulation layer. In yet another aspect the invention relates to a strippable semiconductive shield comprising a chlorinated polyolefin.
2. Description of the Related Art
The construction of insulated, electrical conductors, i.e. wires and cables, designed for all forms of voltage applications but especially for medium to high voltage applications, is well known in the art and commonly comprises a core conductor of one or more strands of a conducting metal or alloy such as copper or aluminum, a layer of semi-conductive shielding, a layer of insulation, such as crosslinked polyethylene, and a layer of semi-conductive insulation shielding, overlying the insulation. A plurality of neutral wires which are usually made of copper may be embedded in or wrapped around the layer of semi-conductive insulation shielding, if desired, in the form of a concentric ring around the insulated cable.
The formation of crosslinking bonds between the insulation and shielding makes subsequent separation of the two layers (insulation and semi-conductive shielding) such as occurs in making splices or terminal connections, very difficult and time consuming. Such a strong bond also makes the semi-conductive layer prone to leave carbon residues on the insulation even when it is finally peeled off. Accordingly, a strippable semi-conductive shielding which can be easily and cleanly stripped from the insulation of an insulated conductor is desirable in this art.
U.S. Pat. No. 4,286,023 teaches that nitrile rubber. e.g., butadiene-acrylonitrile rubber (NBR) is a key component in certain commercial strippable insulation shields as a strip control agent. However, the NBR can be a source of issues such as agglomeration in the rubber handling system in the semiconductive shield production unit, product quality variation from supplier, and property deterioration upon heat aging.
U.S. Published Patent Application 2002/032258 teaches the incorporation of silicon rubber and polypropylene with ethylene-alkyl acrylate copolymer for strippable force control within the polyolefin substrate.
U.S. Pat. No. 6,514,608 teaches a semiconductive jacket material for jacketing a cable, the semiconductive jacket material comprising: (A) a minor phase material comprising a semicrystalline polymer and a conductive filler material dispersed in said minor phase material in an amount sufficient to be equal to or greater than an amount required to generate a continuous conductive network in said minor phase material; and (B) a major phase material, the major phase material comprising a polymer which when mixed with the minor phase material will not engage in electrostatic interactions that promote miscibility. The minor phase material is dispersed within the major phase material. Suitable minor phase materials include any semicrystalline polymer such as high density polyethylene (HDPE), polypropylene, poly-1-butene, poly(styrene) (PS), polycarbonate, poly(ethylene terephthalate), nylon 66, nylon 6 and mixtures of two or more of these polymers. Suitable materials for the major phase material include, for example, ethylene vinyl acetate (EVA), polybutylene terephthalate, PS, poly(methyl methacrylate) (PMMA), polyethylene, polypropylene, polyisobutylene, poly(vinyl chloride), poly(vinylidene chloride), poly(tetrafluoroethylene), poly(vinyl acetate), poly(methyl acrylate), polyacrylonitrile, polybutadiene, poly(ethylene terephthalate), poly(8-aminocaprylic acid), poly(hexamethylene adipamide) and mixtures of two or more of these polymers.